High-frequency notch-excited antenna

ABSTRACT

A high-frequency, high-power transmitting antenna, suitable for use in aerospace vehicles flying at very high altitudes, is described. The antenna assembly comprises an inductive coil with appropriate tuning capacitors installed within a notch in the airframe. The airframe is used as the radiating element and the matching unit is enclosed in a sealed gas-filled container pressurized to approximately one standard atmosphere for the purpose of inhibiting corona in the region of the currentcarrying parts at extremely high altitudes.

OLP'U3"73 XR 397259941 United States Patent 91 [111 3,725,941

Stang [451 Apr. 3, 1973 [54] HIGH-FREQUENCY NOTCH-EXCITED FOREIGN PATENTS OR APPLICATIONS ANTENNA 704,659 2/1954 Great Britain ..343/746 [75] Inventor: Paul F. Stang, Saugus, Calif. Y

[73] Assignee: Lockheed Aircraft Corporation, Primary Tubbeslng Burbank Calif- Attorney-Ralph M. Flygare and George C. Sullivan [22] Filed: Apr. 2, 1968 [21] Appl. No.: 718,034

[57] ABSTRACT A high-frequency, high-power transmitting antenna, suitable for use in aerospace vehicles flying at irery [52] U.S. Cl. ..343/708,343/746,343/789, high altitudes, is described. The antenna assembly 343/872 comprises an inductive coil with appropriate tuning [51] Int. Cl. ..H0lq 1/28 capacitors installed within a notch in the airframe.

[ Field of Search The airframe is used as the radiating element and the 343/861, 789 matching unit is enclosed in a sealed gas-filled container pressurized to approximately one standard at- [56] Rderences Cited mosphere for the purpose of inhibiting corona in the UNITED STATES PATENTS region of the current-carrying parts at extremely high altitudes. 2,781,512 2/l957 Robinson, Jr ..343/86l X 2,908,000 10/1959 Robinson, Jr ..343/708 X 11 Claims, 3 Drawing Figures PATENIEUAPM ms 5,9

sum 1 or 2 INVENTOR. PAU L F. STANG Agents TRANSMITTER I I I l FIG. 2

f I i HIGH-FREQUENCY NOT CE-EXCITED ANTENNA BACKGROUND OF THE INVENTION It has been found that relatively low radio-frequency (RF) potentials (e.g., 300 volts) are sufficient to cause 5 corona to appear in antenna areas at low-pressure regions as exist at the very high altitude (e.g., 200,000 feet) environments in which aerospace vehicles operate. This environment makes it impractical to use conventional capacitive-type antennas driven with the high voltages which are characteristic of high-frequency (HF) or very-high-frequency (VHF) transmitters. These limiting properties of prior antennas are overcome in the present invention by means of a novel lowvoltage, high-current antenna of the inductive type. A notch-coupled high-frequency antenna is an example of a low-voltage, high-current antenna ofthe prior art and such devices have been used successfully heretofore in aircraft designed to fly at altitudes below 50,000 feet. However, this type of antenna may be used in a high altitude environment (e.g., 50,000 to 200,000 feet) and at high power levels (e.g., l kilowatt) by incorporating the features of the present invention into their construction.

A notch antenna excites high circulating RF currents in the structure containing the notch. Tail fin excitation, for example, will cause high circulating currents in the region of the notch which spread to the entire airframe and cause it to radiate.

A resonant inductance-capacitance circuit, combined with the radiation resistance of the excited structure of the surrounding airframe, provides the desired low impedance. For maximum power transfer the small input impedance of the notch circuit must be transformed through a matching circuit to a conventional transmission line (e.g., 50 ohm feedline) which connects the antenna to the transmitter. For high efficiency a tight coupling must be maintained in the antenna input between the matching circuit and resonant circuit of the notch. The degree of coupling determines how effective the high current flow on the airframe surfaces will be. Characteristically, the field around the conductive component .of the notch circuit is low. However, high potential exists-in the impedance matching unit thereby tending to cause corona. The undesired effect of corona or glow discharge is prevented by the present invention wherein the impedance matching unit is contained within a gas-tight dielectric container filled with air or other suitable gas at a pressure of approximately one atmosphere. In a practical construction, the invention has been found to permit a transmitter power of 1 kilowatt to be loaded successfully into the matched antenna at altitudes as high as 200,000 feet, without the adverse effects of corona appearing.

Aside from its above-described advantages, the antenna of the present invention is ideally suited for aerospace vehicles use since it may be flush-mounted in the surface of the airframe so as not to affect adversely the aerodynamic properties of the vehicle. Other advantages will become apparent from the ensuing portion of this specification.

It is therefore an object of the invention to provide a novel and improved antenna having very high altitude performance suitable for aerospace vehicular use.

It is another object of the invention to provide a novel and improved notch exciter for the efficient 2 radiation of electromagnetic energy at high power levels.

Another object of the invention is to provide a pressurized notch-excited antenna having a greater efficiency than generally similar antennas heretofore used.

Still another object of the invention is to provide a novel and improved transmitting antenna which will operate at high driving powers and which is substano tially immune to corona discharge or voltage breakdown.

Yet another object of the invention is to provide a matching unit for a transmitting antenna, which assembly is contained within a sealed enclosure affording protection from corrosion and adverse environmental effects.

A general object of the invention is to provide an antenna having performance superior to antennas of the same general type employed heretofore.

BRIEF DESCRIPTION OF THE INVENTION FIG. 1 is a fragmentary side elevation view showing the empennage of an aircraft incorporating an antenna constructed in accordance with the invention.

FIG. 2 is a schematic diagram illustrating the circuit components of the invention.

FIG. 3 is a somewhat diagrammatic side elevation view showing the antenna of FIG. 1 in greater detail.

Although the antenna of the present invention may be employed wherever its novel features are desired, a typical application is its use as the HF transmitting antenna for an aerospace vehicle. Accordingly, this application will be described in detail as an illustrative embodiment of the invention.

Referring first to FIG. 1, there is shown the empennage portion of an airplane in which the antenna of the present invention is installed. The antenna notch essentially comprises an electrical discontinuity in the metal airframe and is preferably reinforced by means of a rigid box-shaped structure 1, open on two opposing sides on one end, preferably constructed of extruded or folded metal channels. The width of the reinforcing metal channel is selected to be substantially the same as the thickness of the vertical stabilizer in which the antenna is installedpThe notch-defining structure 1 is.

cover 3 which is flush mounted with respect to the skin 2 of the aircraft, thereby maintaining the desired external contour of the airframe.

In FIG. 1, the open end of the notch (viz., the nonconductive end) is placed at the leading edge of the vertical stabilizer 4 of the airplane. The notch extends, transversely, entirely through the vertical stabilizer 4. A pressurized matching unit 5 is contained within the area of the notch.

' The circuit elements comprising the antenna and its impedance matching'unit are schematically shown in FIG. 2. The top side (viz., upper conducting edge) of the notch is indicated at 6 and corresponds to one of the conducting edges of the aircraft skin 2 and its supporting structure 1. The closed circuit path terminating along the entire peripheral edge of the notch includes a resistive component 7 and an inductive component 8.

RF energy from the transmitter is fed to the antenna through a coaxial cable or other suitable transmission line comprising conductors 9 and 11. The central conductor 11 of the cable passes through an aperture in the notch-forming structure 1 and is coupled to the upper end of coil 13 comprising the antenna matching assembly, as can best be seen in FIG. 2. The outer conductor 9 or shield of the transmission cable is connected to the lower edge of the notch structure near the leading edge, via conductor 12. The matching assembly is so arranged as to effect a proper impedance match between the transmission feedline and the notch. The

lower end of coil 13 is coupled via variable capacitor 14 to the second (lower) side of the notch. Coil 13 is inductively coupled to conductor 15 having its upper terminus at the first side (6) of the notch. Fixed capacitor 16 and variable capacitor 17 are capacitively coupled to the lower terminus of conductor 15 and the second edge 18 of the notch.

Trimming adjustments for matching the impedance of the antenna coupler to the characteristic impedance of the feedline may be made by means of variable capacitor 14.

The width and depth of the notch cut-out of the airframe determines the inductive component 8. The effective radiation resistance (8) is in series with the notch inductance (7) and the tuning capacitance. At the lower end of the HF band the radiation resistance typically may be of the order of 0.1 ohms. For this reason, the overall efficiency of the antenna is dependent upon the conductivity of the metal skin (2).

The notch circuit inductance is preferably of the order of 1.5 microhenry for an antenna in the HF communications range (2 MHz to 30 MHz). The capacitor bank comprising capacitors 16 and 17 is preferably installed at a location as close as physically practical to the antenna input. That is, conductor 12 and notch edge 18 should be closely connected, and the capacitor bank connected to their common junction. Since the impedance of any practical antenna varies widely over the design frequency range, and the matching requirements for maximum radiated power are stringent, it is impractical to use a single broadband matching configuration, and the antenna must be matched by a network with its elements appropriately adjusted at each frequency. By means of appropriate adjustments in the capacitance of the capacitor bank, in a manner to be described more fully hereinafter, the antenna may be made to match the selected transmission frequency and large circulating currents will be caused to flow around the area of the notch.

The input impedance of the notch becomes a pure resistance when tuned to resonance at one frequency. The value of the capacitance in series with the inductive component of the circuit is chosen to be able to resonate the circuit. The following equation describes 'thenotch radiation resistance:

R,= 120 (96) ll).

where:

l= length of slot A wavelength As I approaches zero at constant A, R, approaches zero, which confirms the validity of I01. The impedance of the notch is a function of l/w, where w is notch width, and is computed from The impedance Z Z,,,, Tan B where B 360/ )trepresents the antenna impedance as seen through the transmission line as a function of frequency and line length.

Assuming that the notch is located in the empennage of the airplane, as shown in FIG. 1, it is preferred that the path length from the notch to the wing tips or nose section of the aircraft should approach a resonant length at the low end of the band. This may be achieved in practice in large aircraft.

The transformation of the notch impedance to the characteristic of the 50 ohm transmission line is achieved by the novel matching network comprising elements 13-17. The input impedance of the notch becomes a pure resistance when tuned to resonance at one frequency. The value of the capacitance (l6 and 17) in series with the inductive component (15) of the circuit is chosen to be able to resonate the circuit.

Referring to FIG. 3 there is shown a typical embodiment of an antenna for installation in the manner shown in FIG. 1. This construction employs a frame comprising members 21-23 which are preferably fabricated from sheet metal which has inwardly folded edges to result in a channel shape providing improved rigidity. The open side of the U-shape structure comprising members 21-23 are provided with flange portions for securing the antenna to a dielectric cover 24. A metal rod 25 extends downwardly from top member 21 and is secured thereto by a suitable fastening means. Capacitors 26-29 are interposed between the lower end of rod 25 and the upper interior surface of member 23. Conductor bar 31 is secured to the lower end of rod 25 and provides a suitable attachment point for the upper terminals of capacitors 26-29. Capacitors 26-28 are fixed capacitors whereas capacitor 29 is of the variable type to provide tuning adjustment.

The shield conductor 32 of the co-axial feedline is electrically connected to member 23 and the inner conductor 33 passes through an insulated aperture in member 23 for connection to coil 34.

The inductive component of the antenna matching unit comprises a plurality of series-connected toroidal coils, the number and total inductance of which is determined by the desired input impedance of the antenna. Typical ones of these coils are identified as 34 and 36. The separate coils are connected in series by appropriate conductors, typical ones of which are identified as 35 and 37. The last coil in the series is connected by conductor 38 to variable capacitor 39. The lower terminal of capacitor 39 is returned to the shield conductor 32 via member 23 at point 41.

To minimize voltage breakdown, the toroidal coil sections (34 and 36) of the matching unit are insulted of l kilowatt at 80,000 feet.

one from the other by means of interposed insulator disks, typical ones of which are identified as 41 and 43. Also, rod 25 is insulated from the coil sections (34 and 36) by insulator sleeve 44.

Insulator caps 45 and 46 comprise disks which enclose and hermetically seal respective ends of dielectric cylinder 47. Disk 46 is provided with sealed feedthrough conductors for lead 33 and the lead interconnecting point 41 with capacitor 39. The sealed interior of cylinder 47 is pressurized with air or other inert gas at a pressure of approximately one atmosphere. This will effectively insulate the electrical components and suppress corona or voltage breakdown when the antenna is operated at high altitudes.

Capacitor 39 need only be adjusted at the time of installation in order to match the co-axial feedline to the matching unit. For this reason it may be contained within the pressurized container (45-47). However, it may be necessary to retune the antenna from time to time in order to accommodate various operating frequencies. This may be accomplished by switching various capacitors into or out of the circuit or otherwise adjusting the total capacitance between bar 31 and point 41. Thus, these capacitors (viz., 26-29) are not enclosed within the pressurized container (45-47). Inasmuch as the potential across capacitors 26-29 is relatively low, it is unnecessary to enclose them with a corona suppression environment as is necessary for capacitor 39.

In a practical construction of the invention, designed to operate at a frequency of 2 megahertz, an input to the antenna of approximately 170 watts was successfully used and a voltage standing wave ratio (VSWR) of less than 1.05:1 was obtained. In this practical embodiment the reflected power was observed to remain below 0.5 watts. Corona discharge did not occur under these operating conditions at altitudes to 250,000 feet. This performance corresponds to a transmitter power Thus, there is provided an antenna meeting the objectives set forth hereinabove and of which is readily adapted to installation in an aerospace vehicle.

While there have been shown and described and pointed out the, fundamental novel features of the invention-as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in the form and details of the device illustrated may be made by those versed in the art. For example, the orientation of the notch structure with respect to the airframe and its enclosing dielectric cover may be altered in accordance with specified aerodynamic parameters, and the slot dimensions may be increased or decreased dependent only on electrical and mechanical design requirements. Such modificationsmay be made without departing from the spirit of the invention; therefore, it is intended that the invention be limited only as indicated by the scope of the following claims.

What is claimed is: 1.An antenna comprising:

a sheet of electrically conducting material having a U-shaped notch therein; tuning means shunted across said U-shaped notch; .impedance matching means inductively coupled to said tuning means;

a transmission feedline having a first conductor connected to said sheet at an edge of said notch and a second conductor connected to said impedance matching means; and,-

a gas-filled dielectric enclosure surrounding said impedance matching means for suppressing corona discharge therein.

2. An antenna as defined in claim 1 wherein said tuning means comprises:

a conductor rod having a first end secured to one side of said notch and extending toward the other side of said notch; and,

selectively variable capacitance means connected in series between said other side of said notch and the second end of said rod.

3. An antenna as defined in claim 2 wherein said matching means comprises:

coil means encircling said rod and inductively coupled thereto; and

a variable capacitor connected in series between said coil means and the junction of said capacitance means and said other side of said notch.

4. An antenna as defined in claim 3 wherein said enclosure comprises:

a substantially cylindrical dielectric envelope having closed ends and surrounding said coil means and said variable capacitor, and disposed with its major axis extending along the major axis of said rod.

5. An antenna as defined in claim 1 wherein said transmission feedline comprises:

a coaxial cable having a shield conductor connected to said other side of said notch and having its inner conductor connected to said impedance matching means.

6. An antenna as defined in claim 1 wherein the gas in said enclosure comprises air at a pressure of approximately one atmosphere.

7. An antenna as defined in claim 1, wherein said coil means comprise:

a plurality of series-connected toroidal coils coaxially disposed in side-by-side relationship along a portion of said rod. 7 I

8. An antenna as defined in claim 7 including first insulator means interposed between said toroidal coils; and,

second insulator means interposed between said to? roidal coils and said rod.

9. An antenna as defined in claim 1 including a dielectric cover extending across said notch to maintain the surface integrity of said sheet.

10. An antenna asdefined in claim 1 wherein said impedance matching means is located-entirely within said notch.

11. A high-frequency notch-excited antenna comprising: l v

a generally U-shaped frame constructed of a-conductive material;

a substantially planar conductive sheet secured to said frame and extending outwardly therefrom to a distance sufficient to provide a path length from said frame which approximates a resonant length for said antenna;

a rod conductor secured at one end to said frame and having its unsecured end extending across a portion of the open end of said frame;

7 8 adjustable capacitor means connected between said a two-conductor transmission line having one of its frame and the unsecured and of said conductors connected to said frame at said junca plurality of series-connected toroidalcoils coaxially disposed with respect to said rod and inductively coupled thereto;

a tuning capacitor connected between one end of said series-connected coils and the junction of said capacitor means and said frame;

tion and the other of its conductors connected to said series-connected coils; and,

a gas-filled envelope enclosing said series-connected coils and said tuning capacitor. 

1. An antenna comprising: a sheet of electrically conducting material having a U-shaped notch therein; tuning means shunted across said U-shaped notch; impedance matching means inductively coupled to said tuning means; a transmission feedline having a first conductor connected to said sheet at an edge of said notch and a second conductor connected to said impedance matching means; and, a gas-filled dielectric enclosure surrounding said impedance matching means for suppressing corona discharge therein.
 2. An antenna as defined in claim 1 wherein said tuning means comprises: a conductor rod having a first end secured to one side of said notch and extending toward the other side of said notch; and, selectively variable capacitance means connected in series between said other side of said notch and the second end of said rod.
 3. An antenna as defined in claim 2 wherein said matching means comprises: coil means encircling said rod and inductively coupled thereto; and a variable capacitor connected in series between said coil means and the junction of said capacitance means and said other side of said notch.
 4. An antenna as defined in claim 3 wherein said enclosure comprises: a substantially cylindrical dielectric envelope having closed ends and surrounding said coil means and said variable capacitor, and disposed with its major axis extending along the major axis of said rod.
 5. An antenna as defined in claim 1 wherein said transmission feedline comprises: a coaxial cable having a shield conductor connected to said other side of said notch and having its inner conductor connected to said impedance matching means.
 6. An antenna as defined in claim 1 wherein the gas in said enclosure comprises air at a pressure of approximately one atmosphere.
 7. An antenna as defined in claim 1, wherein said coil means comprise: a plurality of series-connected toroidal coils coaxially disposed in side-by-side relationship along a portion of said rod.
 8. An antenna as defined in claim 7 including first insulator means interposed between said toroidal coils; and, second insulator means interposed between said toroidal coils and said rod.
 9. An antenna as defined in claim 1 including a dielectric cover extending across said notch to maintain the surface integrity of said sheet.
 10. An antenna as defined in claim 1 wherein said impedance matching means is located entirely within said notch.
 11. A high-frequency notch-excited antenna comprising: a generally U-shaped frame constructed of a conductive material; a substantially planar conductive sheet secured to said frame and extending outwardly therefrom to a distance sufficient to provide a path length from said frame which approximates a resonant length for said antenna; a rod conductor secured at one end to said frame and having its unsecured end extending across a portion of the open end of said frame; adjustable capacitor means connected between said frame and the unsecured end of said rod; a plurality of series-connected toroidal coils coaxially disposed with respect to said rod and inductively coupled Thereto; a tuning capacitor connected between one end of said series-connected coils and the junction of said capacitor means and said frame; a two-conductor transmission line having one of its conductors connected to said frame at said junction and the other of its conductors connected to said series-connected coils; and, a gas-filled envelope enclosing said series-connected coils and said tuning capacitor. 